This invention relates to an inertia switch having an operating threshold which is dependent upon the speed of rotation of the switch.
Inertia switches are known and are frequently used in braking system of motor vehicles to prevent the wheels of the vehicle locking during braking.
The known switches, such as that disclosed in U.S. Pat. No. 2,972,027, generally consist of a flywheel driven by an element mounted on a shaft which is rotated at a speed depending upon the rotational speed of a vehicle wheel. Deceleration of the wheel as a result of braking causes the flywheel to pivot relative to the drive element and to close the contacts of an electrical switch, which contacts are respectively connected to the drive element and the flywheel. Before the flywheel can pivot forwards relative to the drive element, the inertia of the flywheel must be greater than a return force provided by a resilient element secured between the flywheel and the drive element. The deceleration of the drive element must therefore be sufficient for this return force to be overcome if the switch is to be actuated. Calibration of the resilient element determines the value of this return force and hence the deceleration threshold below which the electrical switch does not close. This makes it possible to distinguish from wheel deceleration not due to deliberate hard braking.
However, with these prior art devices the value of the deceleration threshold is fixed and is independent of the speed of rotation of the device and hence of the wheel. Therefore, particularly at low speeds, because the inertia of the flywheel may be insufficient to overcome the return force of the resilient means, the wheel can become locked without the switch being actuated.
During braking at high or medium speeds the switch may be actuated several times. While the inertia of the flywheel is variable, because it depends on the speed of rotation, which, of course, decreases during braking, the return force provided by the resilient element remains constant. Consequently, the response time of the switch increases as the vehicle slows down, and the frequency at which the switch is actuated decreases accordingly, thereby preventing optimum braking.